1. Field of the Invention
In general, the invention pertains to a method for controlling the pyrolysis of a complex waste stream of plastics to convert the stream into useful high value monomers or other chemicals, thereby minimizing disposal requirements for non-biodegradable materials and conserving non-renewable resources. The method uses fast pyrolysis for sequentially converting a plastic waste feed stream having a mixed polymeric composition into high value monomer products by:
using molecular beams mass spectrometry (MBMS) techniques to characterize the polymeric components of the feed stream and determine process parameter conditions; PA1 catalytically treating the feed stream to affect the rate of conversion and reaction pathways to specific products; and PA1 differentially heating the feed stream containing catalyst according to a heat rate program using predetermined MBMS data to sequentially obtain optimum quantities of high value monomer and other high value products from the selected components in the feed stream. PA1 using molecular beam mass spectrometry (MBMS) to characterize the components of the feed stream; PA1 catalytically treating the feed stream to affect the rate of conversion and reaction pathways to be taken by the feed stream leading to specific products; PA1 selection of coreactants, such as steam or methanol in the gas phase or in-situ generated HCl; and PA1 differentially heating the feed stream according to a heat rate program using predetermined MBMS data to provide optimum quantities of said high value monomer products or high value chemicals.
From the conditions selected using the MBMS, batch or continuous reactors can be designed or operated to convert mixed plastic streams into high value chemicals and monomers.
The invention achieves heretofore unattained control of a pyrolysis process, as applied to mixed polymeric waste, through greater discovery of the mechanisms of polymer pyrolysis, as provided through the use of molecular beam mass spectrometry. Pyrolysis mass spectrometry is used to characterize the major polymers found in the waste mixture, and the MBMS techniques are used on large samples in a manner such that heterogeneous polymeric materials can be characterized at the molecular level. After characterization, in accordance with the method of invention, when a given a specific waste stream polymer mixture, that mixture is subjected to a controlled heating rate program for maximizing the isolation of desired monomer and other high value products, due to the fact that the kinetics of the depolymerization of these polymers have been determined as well as the effects of catalytic pretreatment which allow accelerating specific reactions over others, thus permitting control of product as a function of catalyst and temperature (heating rate).
2. Description of the Prior Art
U.S. Pat. No. 3,546,251 pertains to the recovery of epsilon-caprolactone in good yield from oligomers or polyesters by heating at 210.degree.-320.degree. C. with 0.5 to 5 parts weight of catalyst (per 100 parts weight starting material) chosen from KOH, NaOH, alkali earth metal hydroxides, the salts of metals e.g. Co and Mn and the chlorides and oxides of divalent metals.
U.S. Pat. No. 3,974,206 to Tatsumi et al. discloses a process for obtaining a polymerizable monomer by: contacting a waste of thermoplastic acrylic and styrenic resin with a fluid heat transfer medium; cooling the resulting decomposed product; and subjecting it to distillation. This patent uses not only the molten mixed metal as an inorganic heat transfer medium (mixtures or alloys of zinc, bismuth, tin, antimony, and lead, which are molten at very low temperatures) alone or in the presence of added inorganic salts, such as sodium chloride, etc., molten at &lt;500.degree. C. but an additional organic heat transfer medium, so that the plastic waste does not just float on the molten metal, and thereby not enjoy the correct temperatures for thermal decomposition (&gt;500.degree. C.). The molten organic medium is a thermoplastic resin, and examples are other waste resins such as atatic polypropylene, other polyolefins, or tar pitch. The added thermoplastic is also partially thermally decomposed into products that end up together with the desired monomers, and therefore, distillation and other procedures have to be used to obtain the purified monomer.
However, since Tatsumi et al. deal with acrylic polymers known to decompose thermally-into their corresponding monomers, the patent provides no means for identifying catalyst and temperature conditions that permit decomposition of that polymer in the presence of others, without substantial decomposition of the other polymers, in order to make it easier to purify the monomer from the easier to decompose plastic or other high-value chemicals from this polymer.
U.S. Pat. No. 3,901,951 to Nishizaki pertains to a method of treating waste plastics in order to recover useful components derived from at least one monomer selected from aliphatic and aromatic unsaturated hydrocarbons comprising: melting the waste plastic, bringing the melt into contact with a particulate solid heat medium in a fluidized state maintained at a temperature of between 350.degree. to 650.degree. C. to cause pyrolysis of the melt, and collecting and condensing the resultant gaseous product to recover a mixture of liquid hydrocarbons; however, even though one useful monomer (styrene) is cited, the examples produce mixtures of components, all of which must be collected together and subsequently subjected to extensive purification. No procedure is evidenced or taught for affecting fractionation in the pyrolysis itself by virtue of the catalysts and correct temperature choice.
U.S. Pat. No. 3,494,958 to Mansfield et al. is directed to a process for thermal decomposition of polymers such as polymethyl methacrylate using the fluidized bed approach, comprising: taking finely divided polymers of grain size less than 5 mm and windsifting and pyrolysing said polymer grains at a temperature which is at least 100.degree. C. over the depolymerization temperature to produce monomeric products; however, this is a conventional process that exemplifies the utility of thermal processing in general for recovery of monomers from acrylic polymers which, along with polytetrafluoroethylene, are the only classes of polymers which have monomers recovered in high yield by thermal decomposition. See, for instance, A. G. Buekens in Conservation and Recycling, Vol. 1, pp. 241-271 (1977). The process of this patent does not acknowledge the need of taking the recovery a step further in the case of more complex mixtures of products, let alone provide a means for doing so.
U.S. Pat. Nos. 4,108,730 and 4,175,211 to Chen et al. relate respectively to treating rubber wastes and plastic wastes by size reducing the wastes, removing metals therefrom, and slurrying the wastes in a petroleum--derived stream heated to 500.degree.-700.degree. F. to dissolve the polymers. The slurry is then fed into a zeolite catalytic cracker operating at 850.degree. F. and up to 3 atmospheres to yield a liquid product, which is a gasoline-type of product.
While the Chen et al. references exemplify catalytic conversion, it is to a mixture of hydrocarbons boiling in the gasoline range, and not to make specific useful compounds(s), which can be formed and isolated by virtue of temperature programming and catalytic conditions.
U.S. Pat. No. 3,829,558 to Banks et al is directed to a method of disposing of plastic waste without polluting the environment comprising: passing the plastic to a reactor, heating the plastic in the presence of a gas to at least the decomposition temperature of the plastic, and recovering decomposition products therefrom. The gas used in the process is a heated inert carrier gas (as the source of heat).
The method of this patent pyrolyses the mixtures of PVC, polystyrene, polyolefins (in equal proportions) at over 600.degree. C., with steam heated at about 1300.degree. C., and makes over 25 products, which were analyzed for, including in the order of decreasing importance, HCl, the main product, butenes, butane, styrene, pentenes, ethylene, ethane, pentane and benzene, among others.
In Banks, no attempt is made to try to direct the reactions despite the fact that some thermodynamic and kinetic data are obtained.
U.S. Pat. No. 3,996,022 to Larsen discloses a process for converting waste solid rubber scrap from vehicle tires into useful liquid, solid and gaseous chemicals comprising: heating at atmospheric pressure a molten acidic halide Lewis salt or mixtures thereof to a temperature from about 300.degree. C. to the respective boiling point of said salt in order to convert the same into a molten state; introducing into said heated molten salt solid waste rubber material for a predetermined time; removing from above the surface of said molten salt the resulting distilled gaseous and liquid products; and removing from the surface of said molten salt at least a portion of the resulting carbonaceous residue formed thereon together with at least a portion of said molten salt to separating means from which is recovered as a solid product, the solid carbonaceous material.
In the Larsen reference, the remainder from the liquid and gaseous fuel products is char. Moreover, these products are fuels and not specific chemicals.
Table 1 summarizes examples from the literature on plastic pyrolysis. TBL3 TABLE 1 Thermal decomposition of polymers (adapted from Buckens) Process developed Reactor type & heating Reaction by method temperature, .degree.C. Plant capacity, tons/day Feedstock Products References a) Union Carbide Extruder, followed by 420-600 0.035-0.07 PE, PP, PS, PVC, PETP, Waxes annular pyrol, tube, PA, mixes electrically heated b) Japan Steel Works Extruder c) Japan Gasoline Co. Tubular reactor, externally Dissolved or suspended in Heavy-oil heated recycle-oil d) Prof. Tsutsumi Tubular reactor, 500-650 1 PS-foam superheated steam as a heat carrier e) Nichimen* Calalytic fixed bed reactor Mixed plast, no char- forming polymers f) Toyo Engineering Fluidized bed catalytic 0.5 Mixed plast., no char- Corp. reactor forming polymers g) Mitsui Shipbuilding & Stirred tank reactor, 420-455 24-30 Low mol. w. polymers (PE, Fuel-oil Engineering Co. polymer bath APP0 h) Mitsui Petrochemical Industries Co. (Chiba Works) i) Mitsubishi Heavy Ind. Tank reactor with 400-500 0.7/2.4 Polyolefins Naphtha kerosene (Mihara Works) circulation pump and fuel-oil reflux cooling j) Kawasaki Heavy Ind. Polymer bath, formed by 400-450 5 Mixed plast. PE + PS Gas-oil HCL (Kakogawa Works) PE and PS content 55% k) Ruhrchemie AG, Stirred tank reactor, salt 380-450 1.2 PE Oil, wax Oberhausen bath l) Japan Gasoline Co. Fluidized bed 450 0.2 PS-waste m) Prof. Sinn, Univ. of Fluidized bed 640-840 Laboratory scale PE, PS, PVC tyre rubber Aromatic hydro- Hamburg Prof. Kaminsky Molten salt bath 600-800 Laboratory scale carbons & fuel oil n) Sanyo Electric Co. Tubular reactor with a 260 (PVC), followed 0.3 (pilot) Foam PS. mixed plast. Monomer screw for carbon removal, by 500-550 3 (Gifu) (select. collect) assphalt Fuel-oil dielectric heating 5 (Kusatsu) 6% S HCL o) Sumitomo Shipbuild. & Fluidized bed, partial 450-470 3-5 Mixed plastics incl. PVC Heavy oil Machinery Co. oxidation 600 (28) HCL (Hiratsuka Lab.) p) Government Industrial Fluidized bed, partial 400-510 Bed diameter: 3.5/ PS-chips Monomer and dimer Research Institute oxidation 550 15/30/50 & 120 cm Gasific. prod. q) Nippon Zeon, Japan Fluidized bed, partial 350-600 24 pre-commercial Sheared tyres Gas, oil, char Gasoline Co. oxidation (400-500 mostly) plant (Tokuyama) r) Kobe Steel Externally heated, rotary 600-800 5 (pilot) Crushed types Gas, oil, char kiln s) Bureau of Mines/ Electrically heated retort 500/900 Laboratory scale Tyre cuttings Gas, oil, char Firestone t) Hydrocarbon Research Autoclave 350-450 Tyres Inc. u) Zeplichal Conveyor band, vacuum Tyres v) Herbold, W. Germany References Modified from A. G. Buekens, "Some Observations On The Recycling of Plastics and Rubber" in Conservation and Recycling, Vol. 1, pp. 247-271 (1977)